This invention relates to a process for purifying dimethyl carbonate which is useful as a synthesis starting material for an aromatic polycarbonate and various chemical products and as a solvent. More particularly, it relates to a process for purifying dimethyl carbonate which comprises distillating and separating methanol to purify dimethyl carbonate by co-presenting dimethyl oxalate in order to prevent reacting carbon monoxide and an ester of nitrous acid in the presence of a specific catalyst with high yield.
As a process for preparing dimethyl carbonate, there have been practiced by, for example, the method of reacting carbon monoxide, methanol and an acid by using copper chloride (Japanese Patent Publications No. 11129/1970 and No. 45655/1980), the method of interesterifying a cyclic carbonate such as ethylene carbonate with methanol in the presence of a catalyst (U.S. Pat. Nos. 3,642,858 and 3,803,201, and Japanese Patent Publication No. 27658/1985), and the method of subjecting to vapor reaction of carbon monoxide and nitrite in the presence of a catalyst (Japanese Unexamined Patent Publications No. 274816/1989 and No. 201146/1990). However, in either of the methods, dimethyl carbonate can be obtained as a mixture with methanol so that separation from methanol is indispensable for purifying dimethyl methyl carbonate. Dimethyl carbonate and methanol constitute azeotropic mixture in a composition ratio of 30:70 (weight ratio), and thus, it is difficult to separate the mixture by distillation under normal pressure.
Thus, many investigations have been carried out about the method for purifying dimethyl carbonate from the mixture of the both components, and various proposals have been made. For example, there has been proposed the method of obtaining crystalline product enriched in dimethyl carbonate by cooling as disclosed in U.S. Pat. No. 3,803,201, the method of subjecting to separation of the mixture by distillation by breaking azeotrope with pressurization as disclosed in Japanese Patent Publication No. 3463/1984, and the method of subjecting to separation by distillation by adding a hydrocarbon such as hexane and heptane as disclosed in Japanese Unexamined Patent Publication No. 41820/1979. However, these methods cannot be said to be industrially satisfied. That is, the method of using extraction and distillation with water is not economical since dimethyl carbonate dissolves well in water and easily saponified so that causing many loss. An apparatus to be used in the method of distillation under pressure becomes extremely high cost and operations thereof are difficult. Also, the method of adding hydrocarbon is complicated in operations and disadvantageous in energy. Further, the method of precipitation by cooling is industrially not practical.